Most artificial satellites, spacecraft and other craft such as aircraft, ship and ground vehicles (collectively referred to herein as vehicles) require information about their locations and/or attitudes to accomplish their missions. This information may be obtained from one or more sources, such as the global positioning system (GPS), ground-based radar tracking stations and/or on-board star trackers.
A star tracker is an optical device that measures angles to one or more stars, as viewed from a vehicle. A star tracker typically includes a star catalog that lists bright navigational stars and information about their locations in the sky, sufficient to calculate a location of a vehicle in space, given bearings to one or more of the stars. A conventional star tracker includes a lens that projects an image of a star onto a photocell, or that projects an image of one or more stars onto a pixelated light-sensitive sensor array (collectively, a digital camera). The lens typically constitutes a large fraction of the mass of the star tracker. The digital camera also typically constitutes a large fraction of the electronics of the star tracker, and it consumes a significant fraction of the electrical power consumed by the star tracker.
One type of star tracker is “strapped-down,” meaning its view angle, relative to its vehicle, is fixed. Another type of star tracker can be aimed mechanically, such as in a direction in which a navigational star is expected to be seen. Using data from the photocell or sensor array, the star catalog and information about the star tracker's view angle, relative to the vehicle, the star tracker calculates a position of the vehicle in space.
Strap-down star trackers are mechanically simpler than mechanically aimable (gimbaled) star trackers. However, the fixed view angle of a strap-down star tracker limits the number of navigational stars that may be used. Mechanically aimable star trackers can use a larger number of navigational stars. However, aiming a star tracker, relative to its vehicle, with the required precision, poses substantial problems.
An ideal star tracker would be mechanically, electrically and optically simple, small, low in mass and consume little power.
Jinkui Chu, et al., describe a polarization-based navigation system for a mobile robot (Design of a Novel Polarization Sensor for Navigation, Proceedings of the 2007 IEEE International Conference on Mechatronics and Automation, Aug. 5-8, 2007, pp. 3161-3166, Harbin, China and Application of a Novel Polarization Sensor to Mobile Robot Navigation, Proceedings of the 2009 IEEE International Conference on Mechatronics and Automation, Aug. 9-12, 2009, pp. 3763-3768, Changchun, China). However, the Chu device requires incoming light to be at least fairly strongly polarized. Rayleigh scattering of sunlight in the atmosphere causes polarization patters in the sky, as observed from earth. The Chu device is designed to operate on earth by observing these polarization patters. Star light is essentially unpolarized, or only very slightly polarized. The Chu device is, therefore, not useful as a star tracker, particularly in space.